Bearing component and method for surface hardening

ABSTRACT

Bearing component having a surface at least one part of which has been surface hardened by induction heating. A cross section of the bearing component through said surface exhibits a hardness H surface  at said surface, and remains substantially equal through a first region, a hardness H core  at the non-hardened core of the bearing component and remains substantially equal through a third region and a transition, second region spanning between the first and third regions. The average hardness in the first region of the harness profile is Y 1  and the average hardness in the third region of the harness profile is Y 3  and the hardness profile in the second region has a section defined as a line between the points where 0&lt;k&lt;2, on the hardness profile. The hardness of said section of said second region decreases on average by less than 50 HCR per mm.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Stage application claiming the benefit of International Application Number PCT/SE2011/000095 filed on 27 May 2011, which claims the benefit of SE Application 1000717-7 Filed on 2 Jul. 2010.

TECHNICAL FIELD

The present invention concerns a bearing component for a rolling bearing or a sliding bearing, and a method for surface hardening at least one part of a surface of such a bearing component

BACKGROUND OF THE INVENTION

Many bearing components are through-hardened or carburized in order to meet technical requirements for different applications. Bearing components may namely be subjected to high contact stresses and wear when in use and thus require high hardness.

Induction hardening is a heat treatment in which a metal component is heated to the ferrite/austenite transformation temperature or higher by induction heating and then quenched. The quenched metal undergoes a martensitic transformation, increasing the hardness and brittleness of the surface of metal component. Induction hardening may be used to selectively harden areas of a bearing component without affecting the properties of the component as a whole.

U.S. Pat. No. 4,949,758 discloses a process for hardening the interior surface of a long (8-32 feet i.e. 244-975 cm), thin-walled (wall thickness ⅛ to ¼ inch, i.e. 3-6 mm), small inside diameter (1¼ to 3¼ inch, i.e. 32 to 83 mm) tubular member. More particularly, it relates to a process involving progressive heating with an internally positioned, electromagnetic induction coil, followed by immediate quenching with a quench ring assembly, to develop a martensitic case on the inner surface of the tube. This method is used to obtain a surface having a hardness of at least 58 HRC and a substantially non-hardened core with a sharp demarcation between the hardened surface and the non-hardened case core.

When surface hardening a surface of a bearing component it is however advantageous to obtain a hardness profile that does not exhibit a sharp demarcation between the hardened surface and the non-hardened core of the bearing component. A sharp demarcation in hardness results in a short transition zone that causes an increased level of tensile residual stresses, while a smooth demarcation between the hardened surface and the non-hardened core, i.e. a transitional region in which the hardness decreases steadily with depth rather than abruptly, minimizes or eliminates any stresses in the material in that region. Additional stresses, which arise from the application when the bearing component is in use can be withstood much better. Such a steadily decreasing hardness profile may be obtained by carburizing the surface of the bearing component.

Carburizing is a heat treatment process in which iron or steel is heated in the presence of another material that liberates carbon as it decomposes. The surface or case will have higher carbon content than the original material. When the iron or steel is cooled rapidly by quenching, the higher carbon content on the surface becomes hard, while the core remains soft (i.e. ductile) and tough.

SUMMARY OF THE INVENTION

An object of the invention is to provide an improved non-through hardened bearing component.

This object is achieved by a bearing component having a flat or non-flat surface, i.e. an interior or exterior surface, at least one part of which has been surface hardened by induction heating. The surface namely comprises a martensitic microstructure produced by induction hardening using an electromagnetic induction coil followed by quenching using a quenching device. A longitudinal or transverse cross section of the bearing component through the surface exhibits a hardness H_(surface) at the surface and a hardness H_(core) at the non-hardened core of the bearing component (i.e. in the non-hardened base metal of the bearing component. The hardness profile of the cross section (measured using the Vicker's Hardness Test or any other suitable method for example) exhibits a first region whose hardness is substantially equal to the hardness H_(surface) at said surface, a third region whose hardness is substantially equal to the hardness ∞re at the non-hardened core of the bearing component and a second region between said first and third regions. The hardness profile in the first region has an average hardness Y₁, and the hardness profile in the third region has an average hardness Y₃. If a line is drawn on the hardness profile in the second region between the points

${\frac{Y_{1} + Y_{3}}{2} + {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k\mspace{14mu} {and}\mspace{14mu} \frac{Y_{1} + Y_{3}}{2}} - {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k}},$

where 0<k<2 and k is a real number, the hardness of the bearing component in the second region determined along the line decreases by less than 50 HRC per mm.

At least one part of the surface of such a non-through hardened bearing component will exhibit increased surface hardness, increased wear resistance and/or increased fatigue and tensile strength. Furthermore, the induction hardening heat treatment used to produce such a bearing component is more energy efficient and cost effective than a carburizing heat treatment and it has a shorter cycling time and provides better distortion control than a carburizing heat treatment. Furthermore, properties, such as the hardness, microstructure and residual stress, of the at least one part of the interior surface may be tailored as desired for a particular application.

According to an embodiment of the invention, k is 1.

According to an embodiment of the invention k is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, or 1.9.

According to an embodiment of the invention the hardness of the bearing component in the second region determined along said line decreases by less than 30 HRC, less than 25 HRC, less than 20 HRC or less than 15 HRC per mm.

According to another embodiment of the invention the hardness H_(surface) at the surface is between 55-75 HRC on the Rockwell scale, preferably between 58-63 HRC.

According to a further embodiment of the invention the hardness H_(core) at the non-hardened core of the bearing component is between 15-30 HRC.

According to another embodiment of the invention the first region extends from the surface to a depth of up to 6 mm below the surface preferably to a depth of 1-4 mm below the surface, i.e. the volume of material of increased hardness may extend to a depth of about 0.5-6 mm below the surface, preferably 1-2 mm below the surface.

According to an embodiment of the invention, the bearing component has a contact surface that allows a relative movement between the bearing component and another component, e.g. a second bearing component.

According to an embodiment of the invention, the contact surface comprises the at least one part that has been hardened.

According to an embodiment of the invention, the at least one part essentially corresponds to the contact surface.

According to an embodiment of the invention, the contact surface may constitute any of a raceway, a sliding surface, or at least a part of a guide flange or similar.

According to an embodiment of the invention the bearing component may be an inner ring.

According to another embodiment of the invention the bearing component may be an outer ring.

According to an embodiment of the invention the bearing component may be a rolling element, for instance a roller or a ball. According to a further embodiment, the roller may be spherical, cylindrical, tapered, conical, or toroidal.

The bearing component may for example be used in automotive, aerospace, mining, wind, marine, metal producing and other machine applications which require high wear resistance and/or increased fatigue and tensile strength. The bearing component may for example be at least part of a roller, plain bearing, bushing, journal bearing, sleeve bearing, slewing bearing or a rolling element bearing, such as a ball bearing or roller bearing. According to a further embodiment of the invention, the rolling bearing may be any one of a cylindrical roller bearing, a spherical roller bearing, a toroidal roller bearing, a taper roller bearing, a conical roller bearing or a needle roller bearing.

According to a further embodiment of the invention the bearing component comprises, or consists of a carbon or alloy steel with an equivalent carbon content of 0.40 to 1.10%, preferably a high carbon chromium steel. For example the bearing component comprises/consists of 50CrMo4 steel, 100Cr6 steel, or SAE 1070 steel.

The present invention also concerns a method for surface hardening at least part of the surface of a bearing component. The method comprises the steps of heating the at least one part of the surface with an electromagnetic induction coil to the ferrite/austenite transformation temperature or higher by induction heating, maintaining the at least one part of the interior surface at that temperature in order to allow for sufficient heat transport below the surface resulting in a sufficient austenitization of the at least one part, and immediately quenching the at least one part of the surface in order to obtain a cross section of the bearing component through the surface which exhibits a hardness H_(surface) at the surface and a hardness H_(core) at the non-hardened core of the bearing component (i.e. in the non-hardened base metal of the bearing component. The hardness profile of the cross section (measured using the Vicker's Hardness Test for example) exhibits a first region whose hardness is substantially equal to the hardness H_(surface) at said surface, a third region whose hardness is substantially equal to the hardness H_(core) at the non-hardened core of the bearing component and a second region between said first and third regions. The hardness profile in the first region has an average hardness Y₁, and the hardness profile in the third region has an average hardness Y₃. If a line is drawn on the hardness profile in the second region between the points

${\frac{Y_{1} + Y_{3}}{2} + {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k\mspace{14mu} {and}\mspace{14mu} \frac{Y_{1} + Y_{3}}{2}} - {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k}},$

where 0<k<2 and k is a real number, the hardness of the bearing component in the second region determined along the line decreases by less than 50 HRC per mm.

In conventional induction hardening, in which the bearing component is heated up quickly and/or heat is now allowed to spread through the component, in-homogeneous austenite is formed. In the method according to the present invention, heat is allowed to spread through the bearing component for a period of 10 seconds, 20 seconds, 30 seconds, 40 seconds, 50 seconds, 60 seconds, 70 seconds or more than 70 seconds so that homogeneous austenite is formed. The expression “in order to allow for sufficient heat transport below the surface resulting in a sufficient austenitization of the at least one part,” is therefore intended to mean for a time period sufficient for homogeneous austenite to form in the at least one part of the surface. By allowing homogeneous austenite to be formed, the formation of a transitional region, in which the hardness decreases steadily with depth below the surface rather than abruptly, is enabled/facilitated.

According to an embodiment of the method the hardness of the bearing component in the second region determined along said line decreases by less than 30 HRC, less than 25 HRC, less than 20 HRC or less than 15 HRC per mm.

According to an embodiment of the method of the invention, k is 1. According to another embodiment of the invention k is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, or 1.9.

It should be noted that the expression “an induction coil” as used throughout this document with reference to the bearing component and method according to the present invention is intended to mean one or more induction coils. A plurality of induction coils operating in the same or a different manner, for example at the same or different frequencies, may for example be used to simultaneously or consecutively heat a plurality of parts of an exterior surface and/or an interior surface of a bearing component, or one or more parts of a plurality of the bearing components. The induction coil(s) may be arranged to surround one or more parts of a bearing component that is to be hardened or the entire mechanical component.

In an embodiment of the method the induction coil is removed from the bearing component, and a quenching device, such as a quench spray or ring, is used to immediately quench the at least one part of the surface that has been heat treated.

In another embodiment of the method the bearing component is removed from the induction coil, and a quenching device, such as a quench spray or ring, is used to immediately quench the at least one part of the surface that has been heat treated.

According to another embodiment of the invention the first region extends from the surface to a depth of up to 6 mm below the surface preferably to a depth of 1-4 mm below the surface.

According to an embodiment of the invention the bearing component may be any one of an inner ring or an outer ring.

According to an embodiment of the invention the bearing component may be a rolling element, for instance a roller or a ball. According to a further embodiment, the roller may be spherical, cylindrical, tapered, conical, or toroidal.

According to an embodiment of the invention the roller may comprise a hole or a bore. In an embodiment of the invention, the hole or the bore may be hardened.

The bearing component may for example be at least part of a roller, plain bearing, bushing, journal bearing, sleeve bearing, slewing bearing or a rolling element bearing, such as a ball bearing or roller bearing. According to a further embodiment of the invention, the rolling bearing may be any one of a cylindrical roller bearing, a spherical roller bearing, a toroidal roller bearing, a taper roller bearing, a conical roller bearing, or a needle roller bearing.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be further explained by means of non-limiting examples with reference to the schematic appended figures where;

FIGS. 1 & 2 show the steps of a method according to an embodiment of the present invention,

FIG. 3 shows a cross section of a bearing component according to an embodiment of the present invention, and

FIGS. 4 & 5 show hardness profiles of a bearing component according to an embodiment of the present invention, and

FIG. 6 shows a comparison of hardness profiles obtained using carburization and induction hardening.

It should be noted that the drawings have not been drawn to scale and that the dimensions of certain features have been exaggerated for the sake of clarity.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 schematically shows a rotationally symmetrical bearing component 10 manufactured from bearing steel, namely a cylindrical roller, in cross section. The roller 10 is, for example, made of 50CrMo4 steel and comprises a uniform circular cross section which extends all the way through the centre of the component in the longitudinal direction thereof.

An electromagnetic induction coil 14 is used to harden at least one part of the exterior 10 a, 10 b of the roller 10. A source of high frequency electricity (about 1 kHz to 400 kHz) is used to drive a large alternating current through the induction coil 14. The relationship between operating frequency and current penetration depth and therefore hardness depth is inversely proportional, i.e. the lower the frequency the greater the hardness depth.

The passage of current through the induction coil 14 generates a very intense and rapidly changing magnetic field, and the part of the exterior surface 10 a, 10 b to be heated is placed within this intense alternating magnetic field. Eddy currents are generated within that part of the exterior surface 10 a, 10 b and resistance leads to Joule heating of the metal in that part of the exterior surface 10 a, 10 b. The exterior surface 10 a, 10 b of the roller 10 is heated to the ferrite/austenite transformation temperature or higher by induction heating and preferably maintained at that temperature for 10-40 seconds.

In order to select the correct power supply it is first necessary to calculate the surface area of the roller to be heated. Once this has been established empirical calculations, experience and/or a technique such as finite element analysis may be used to calculate the required power density, heating time and generator operating frequency.

In the illustrated embodiment the induction coil 14 is then removed and a quenching device 16, such as a quench spray or ring is used to immediately quenching the at least one part of the exterior surface 0 a, 10 b that has been heat treated. The at least one part of the exterior surface 10 a, 10 b may for example be quenched to room temperature (20-25° C.) or to 0° C. or less. The quenching device 16 is arranged to provide a water-, oil- or polymer-based quench to the heated exterior surface layer 10 a, 10 b whereupon a martensitic structure which is harder than the base metal of the roller 10 is formed. The microstructure of the remainder of the roller 10 remains essentially unaffected by the heat treatment and its physical properties will be those of the bar from which it was machined.

As an example, a 60-200 kW power supply, a frequency of 20-60 kHz, preferably 10-30 kHz or 15-20 kHz a total heating time of 10-40 seconds and a quenching rate and time of 2001/min and quenching time of 40-70 s respectively may be used to obtain a bearing component according to the present invention.

FIG. 2 shows the position of the quenching device 16 while quenching is taking place. It should be noted that at least one other part of the outside surface 10 a, 10 b of the roller 10 may alternatively be subjected to another surface hardening heat treatment, such as induction hardening, flame hardening or any other conventional heat treatment.

Furthermore, even though the roller 10 in the illustrated embodiment has been shown in a horizontal position with the induction coil 14 and quenching device 16 being inserted horizontally, it should be noted that the roller 10 may be oriented in any position. An induction coil 14 and quenching device 16 may for be moved vertically into place from the same or different ends of the roller 10. An induction coil 14 may for example be vertically lowered into place and a quenching device may be vertically raised as the induction coil 14 is withdrawn by raising it vertically.

FIG. 3 shows a longitudinal cross section of the roller 10 after the heat treatment. Part 18 of exterior surface material 10 a, 10 b of the roller 10 comprises a martensitic microstructure produced by induction hardening using an electromagnetic induction coil 14 followed by immediate quenching using a quenching device 16.

The method according to the present invention results in the formation of a transition zone visible in both hardness and in microstructure. The heat treated part 18 of the exterior surface material 10 a, 10 b may namely have a hardness within the range of 55-75 HRC on the Rockwell, preferably 59-63 HRC. The volume of material of increased hardness 18 may for example extend to a depth of up to 6 mm. Such a roller 10 may be used for any application in which a part of the exterior surface 10 a, 10 b is subjected to increased wear, fatigue or tensile stress.

FIG. 4 shows a hardness profile 22 of a longitudinal cross section of a bearing component according to an embodiment of the invention measured radially through a surface hardened region 18 in the direction of arrow 20 shown in FIG. 3. The hardness profile 22 exhibits a first region 24 whose hardness is substantially equal to the hardness H_(surface) at the outer surface 10 a, 10 b of the bearing component 10, between 55-75 HRC, preferably between 58-63 HRC for example. The hardness profile 22 also comprises a third region 26 whose hardness is substantially equal to the hardness H_(core) at the non-hardened core 10 c of the bearing component 10, between 15-30 HRC for example. The hardness profile 22 also comprises a second region 25 between said first region 24 and the third region 26 in which the hardness profile exhibits a smooth transition between the hardness of the first region 24 and the third region 26, i.e. the hardness profile exhibits a transitional region in which hardness decreases steadily with depth below the surface rather than abruptly. The hardness profile in the first region 24 has an average hardness Y₁, and the hardness profile in the third region 26 has an average hardness Y₃, and if a line is drawn on the hardness profile in the second region 25 between the points

${\frac{Y_{1} + Y_{3}}{2} + {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k\mspace{14mu} {and}\mspace{14mu} \frac{Y_{1} + Y_{3}}{2}} - {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k}},$

where 0<k<2 and k is a real number, the hardness of the bearing component in the second region determined along the line decreases by less than 50 HRC per mm.

The depth of the first region 24 and second region 25 may be chosen depending on the application in which the bearing component 10 is to be used.

The dashed line in FIG. 4 shows a hardness profile 30 having a sharp demarcation between the hardness H_(surface) at the outer surface 10 a, 10 b of the bearing component 10 and the hardness H_(core) at the non-hardened core 10 c of the bearing component 10 in which the hardness decreases by more than 50 HRC per mm.

FIG. 5 shows a hardness profile 22 obtained using the method according to the present invention and determined from measured hardness values (measured using Vicker's Hardness Test or any other suitable method). The values may be extrapolated to a depth of 0 mm in order to obtain the hardness H_(surface) at the outer surface 10 a, 10 b of the bearing component 10. In the illustrated embodiment the hardness of the bearing component 10 at a depth of 6-8 mm below the surface of the bearing component 10 may be considered to be the hardness H_(core) at the non-hardened core 10 c of the bearing component 10.

FIG. 6 shows a comparison between hardness profiles obtained using carburization 24, conventional induction hardening 26 and the method according to the present invention 22. It can be seen that the hardness profile resulting from conventional induction hardening 26 decreases abruptly in the transitional region corresponding to the second region 25, as shown in FIG. 4. It can also be seen that the hardness profile obtained using the method according to the present invention 22 decreases much more steadily with depth.

Rather than moving an induction coil and/or quenching device into position relative to a stationary bearing component, a bearing component may be moved into position relative to a stationary induction device and/or quenching device. 

1. A bearing component having a surface, wherein at least one part of said surface has been surface hardened by induction heating, whereby a cross section of the bearing component through said surface exhibits a hardness H_(surface) at said surface and a hardness H_(core) at the non-hardened core of the bearing component, wherein the hardness profile of said cross section exhibits: a first region whose hardness is substantially equal to the hardness H_(surface) at said surface, a third region whose hardness is substantially equal to the hardness H_(core) at the non-hardened core of the bearing component and a second region between said first and third regions, wherein the hardness profile in the first region has an average hardness and the hardness profile in the third region has an average hardness Y₃, and whereby if a line is drawn on the hardness profile in said second region between the points ${\frac{Y_{1} + Y_{3}}{2} + {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k\mspace{14mu} {and}\mspace{14mu} \frac{Y_{1} + Y_{3}}{2}} - {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k}},$ where 0<k<2, the hardness of said bearing component in said second region determined along said line decreases by less than 50 HRC per mm.
 2. The bearing component according to claim 1, characterized in that the hardness of said bearing component in said second region determined along said line decreases by one of: less than 30 HRC per mm, less than 25 HRC per mm, less than 20 HRC per mm, and less than 15 HRC per mm.
 3. The bearing component according to claim 1, wherein said hardness H_(surface) at said surface is between 55-75 HRC.
 4. The bearing component according to claim 1, wherein the hardness H_(core) at the non-hardened core of the bearing component is between 15-30 HRC.
 5. The bearing component according to claim 1, wherein said first region extends from the surface to a depth of up to 6 mm below said surface.
 6. The bearing component according to claim 1, wherein said bearing component is formed into one of an inner ring, an outer ring and a rolling element.
 7. The bearing component according to claim 1, wherein said bearing component is integrated into at least part of a rolling element bearing, a roller bearing, a plain bearing, a bushing, a journal bearing, and a sleeve bearing.
 8. A method for surface hardening at least part of the surface of a bearing component, the method comprising steps of: heating said at least one part of the surface with an electromagnetic induction coil to at least a ferrite/austenite transformation temperature by induction heating, maintaining said at least one part of said surface at said at least a ferrite/austenite transformation temperature in order to allow for sufficient heat transport below the surface resulting in a sufficient austenitization of the at least one part, and immediately quenching said at least one part of the surface in order to obtain a cross section of the bearing component through said surface which exhibits a hardness H_(surface) at said surface and a hardness H_(core) at the non-hardened core of the bearing component, wherein the hardness profile of said cross section exhibits: a first region whose hardness is substantially equal to the hardness H_(surface) at said surface, a third region whose hardness is substantially equal to the hardness H_(core) at the non-hardened core of the bearing component and a second region between said first and third regions, wherein the hardness profile in the first region has an average hardness Y₁, and the hardness profile in the third region has an average hardness Y₃, and whereby if a line is drawn on the hardness profile in said second region between the points ${\frac{Y_{1} + Y_{3}}{2} + {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k\mspace{14mu} {and}\mspace{14mu} \frac{Y_{1} + Y_{3}}{2}} - {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k}},$ where 0<k<2, the hardness of said bearing component in said second region determined along said line decreases by less than 50 HRC per mm.
 9. The method according to claim 8, wherein said hardness of said bearing component in said second region determined along said line decreases by at least one of: less than 30 HRC per mm, less than 25 HRC per mm, less than 20 HRC per mm, and less than 15 HRC per mm.
 10. The method according to claim 8, further comprising a step of maintaining said at least one part of the interior surface at said temperature for one of: at least 10 seconds, 20 seconds, 30 seconds, 40 seconds, 50 seconds, 60 seconds, 70 seconds, and greater than 70 seconds.
 11. The method according to claim 8, further comprising a step of generating said hardness profile of said cross section that exhibits: a first region whose hardness is substantially equal to the hardness H_(surface) at said surface, a third region whose hardness is substantially equal to the hardness H_(core) at the non-hardened core of the bearing component and a second region between said first and third regions in which the hardness profile exhibits a smooth transition between the hardness of said first and third regions.
 12. The method according to claim 11, wherein said first region extends from the surface to a depth of up to 6 mm below said surface.
 13. The method according to claim 8, further comprising a step of forming said bearing component into any one of: an inner ring, an outer ring and a rolling element.
 14. The method according to claim 8, further comprising a step of integrating said bearing component into one of a rolling element bearing, a roller bearing, a plain bearing, a bushing, a journal bearing and a sleeve bearing.
 15. The bearing component according to claim 1, wherein the hardness H_(surface) at said surface is between 58-63 HRC.
 16. The method according to claim 11, wherein said first region extends from the surface to a depth of 1-4 mm below said surface. 